Qualities such as softness, porosity, and biocompatibility have long been sought in the field of vascular prosthesis.
The development of vascular prostheses has been a subject of extensive work over the last 25 years. Most synthetic vascular prostheses have been products of the application of textile technology in this field and have been woven or knitted tubular structures designed to resemble the softness and flexibility of the natural blood vessels. The two major synthetic polymers used as vascular prostheses have been Dacron polyester and polytetrafluoroethylene (PTFE).
These woven or knitted tubular structures are porous and have been produced with smooth or velour surfaces. The porosity, which has been claimed to play a determinant role for the healing process of the arterial prostheses, can be controlled to some extent by adjusting the thread size, or the interstices size, and by the texturization or knit pattern of the synthetic fabric. High degrees of porosity may lead to excessive blood loss after implantation; the velour has been claimed to have the advantage that it fills the interstices of the underlying fabric, thus reducing implant bleeding without reducing the porosity of the implant.
The DeBakey Ultra-light-weight Knitted Prosthesis (USCI, Inc.), the Cooley Graft and the Wesolowski Weavenit (Meadox, Inc.) and the Microknit (Golaski Lab, Inc.) are "smooth wall" commercial Dacron arterial prostheses of different geometric and compositional configurations; the Sauvage Filamentous Velour Prosthesis (USCI, Inc.) is an example of a velour Dacron vascular prosthesis material.
Woven PTFE prostheses have low porosity and have not been used as much as the Dacron prostheses. Also, it has been suggested that their use should be temporary, their permanent use being "dangerous". More recently, "expanded" PTFE, called "Gore-tex" (W. L. Gore & Assoc., Inc.) has become available. "Gore-tex" is a network of small nodules interconnected by thin fibrils and with an adjustable porosity from 0 to 96%, and it has been used with generally encouraging results. Studies with smooth vascular prostheses of expanded PTFE (85% porosity) and with ultra-light weight woven PTFE showed that the patency (i.e., openness or non-occlusion) of the expanded PTFE prostheses was significantly longer (4.5-10 months) in comparison to the woven PTFE prostheses, which occluded in 101 days. It was also demonstrated that the porosity played a critical role in the healing process.
A recent theoretical calculation of the porosity in woven fabrics shows that although their porosity can be designed to vary over a wide range, the existing woven vascular prostheses, e.g. Woven Cooley and Woven DeBakey prostheses have had a low degree of porosity which might not permit complete healing, particularly in small diameter, low-blood-flow locations where the 5 year patency rate is less than 30%.
Ultra-high-molecular-weight polyethylene (UHMWPE) is another polymer which has attracted the interest of many workers for the preparation of artificial prostheses, particularly the construction of orthopedic joint devices, because of its outstanding abrasion resistance and strength. UHMWPE, in contrast to the conventional high-density polyethylenes having average molecular weights up to approximately 400,000, has an extremely high molecular weight, typically 2-8 million, and is intractable. The polymer is supplied as fine powder and is processed into various profiles using compression molding and ram extrusion processes. The intractability of the polymer can be overcome by varying the degree of material cohesion and its initial morphology, more specifically by the formation of gel states and single-crystal mat morphologies and by heating the polymer melt to high temperature ranges, under inert conditions, in which the viscosity of the melt is reduced significantly for melt processing. The preparation of UHMWPE gel states and single crystal mat morphologies have been pursued predominantly for the development of ultra-high modulus and strength fibers. Melt processing of UHMWPE at high temperatures under inert conditions has been investigated for the development of melt-crystallized morphologies with enhanced mechanical properties which may result from the material cohesion which is achieved by processing under such conditions.
Although the prior art covers to a large extent the preparation of superstrong UHMWPE fibrous morphologies by spinning processes which involve a gel intermediate, the focus of the works has been mainly on the development of filamentary products with high modulus and strength in one direction. Also, there has been an expressed desire that such filamentary products have reduced porosity, because porosity may have an adverse effect on the effective transmission of load within the oriented filamentary products. On the contrary, the development of products with bulk properties or enhanced isotropic mechanical properties from gel-like precursors has received no attention. Furthermore, little effort has been devoted to determining the effect of the morphology of the gel-like precursor on the physical properties and the deformability of the products from gel-like precursors. These areas fall within the scope of my invention and they have a potential impact on the production of biomedical devices such as vascular and orthopedic prostheses and sutures and also on the fabrication of profiles possessing the outstanding wear properties of UHMWPE.
An example of a stretched UHMWPE fiber and a process for making it is U.S. Pat. No. 4,413,110. There a slurry of polymer in paraffin oil is heated to between 180.degree. and 250.degree. C., preferably 200.degree.-240.degree. C. and is then cooled to a temperature between -40.degree. C. and +40.degree. C., the paraffin oil being replaced by a more volatile solvent at a temperature below 50.degree. C. and the cooling being rapid and done in such a way as to produce a "gel fiber". This "gel fiber" is then treated to evaporate the more volatile solvent and to stretch the "xerogel" fiber, as it is called, at 120.degree. C. to 160.degree. C., preferably above 135.degree. C. The porosity of the resultant fiber is stated to be "no more than about 10% (preferably no more than about 6% and more preferably no more than about 3%)".